The Verge: "Stutters and slowdowns are plentiful, apps take a long time to open and load"
Android Police: "In terms of raw speed, I would not say the Nexus 9 is slow. The problem is more that it jumps and pauses when doing certain things, and it makes for a jarring user experience in certain situations. [Compared to the Nexus 7 the] Nexus 9 is significantly quicker if an app is already in memory, and the launcher definitely felt a bit snappier"
Phone Arena: "Without question, the Nexus 9 screams with its operations – one that rarely exhibits any sort of delayed response or choppiness. Normal, baseline tasks are all handled with tight responses, but the Nexus 9 delivers great handling with mobile gaming as well. Obviously, the benchmark scores give us a telling tale that it’s a beast."
Engadget: "the Nexus 9 can and will handle just about anything you throw at it. As I made abundantly clear in the software section, the Nexus 9 runs incredibly smoothly while you poke around the OS and launch apps."
Ars Technica: "However, I noticed a fair number of stutters when jumping from task to task, like when starting a new Chrome instance or quitting out of a high-intensity app and waiting for menus and icons to pop back on the screen. "
Conclusion: either very fast or very slow.
I agree with the author's conclusion: "if you asked this lot the time of day you'd get five different answers and one of those wouldn't even be a time."